If your iPhoto is full of pics that you don't need and you want to free up some space on your hard drive, here's a quick fix that will instantly increase your storage space. When you are in iPhoto, click on the images you want to delete in the photo library. Most people make the common mistake of deleting images in an album that only deletes them from that album and not the entire library.
Don't ask me why but pictures are copied to albums, not moved, so essentially you will have two copies of a picture if it's in an album. Once you delete certain pics from your iPhoto library, make sure you empty the trash can as well to permanently delete!